Stabilized polyvinyl nitrate and process

ABSTRACT

A novel stabilized polyvinylnitrate and a process for the production thereof including the precipitation of the compound from organic solutions containing same.

United States Patent Daume et a1.

STABILIZED POLYVINYL NITRATE AND PROCESS Eduard Daume; JurgBreitenmoser, both of Zurich, Switzerland WerkzeugmaschinenfabrikOerlikon-Buhrle AG, Zurich, Switzerland Filed: Sept. 11, 1969 Appl. No.:857,234

Inventors:

Assignee:

Foreign Application Priority Data Sept. 19, 1968 Switzerland 14042/68US. Cl. ..260/45.9 R, 260/77.5 A, 260/78.4 R,

260/791, 260/913 VA, 260/942 R, 260/220, 149/88 [4 1 June 13, 1972Primary Etaminer.loseph L. Schofer Assistant Examiner-Stanford M. LevinAtt0rney--Wenderoth, Lind & Ponack [5 7] ABSTRACT A novel stabilizedpolyvinylnitrate and a process for the production thereof including theprecipitation of the compound from organic solutions containing same.

12 Claims, No Drawings STABILIZED POLYVINYL NITRATE AND PROCESS Thepresent invention relates to stabilized polyvinylnitrate (PVN), and tomethods for producing same.

In the development of fuels for rocket motors, it is endeavored toproduce fuel compositions which are as energyrich as possible, in orderthereby to be ableto deliver large payloads with small volumes of fuel.In the field of solid-fuel rockets, the so-called double-base" fuels aremainly used to this end. They consist essentially of a mixture ofnitrocellulose and nitroglycerine, to which additives are admixed formodifying the combustion properties. Moreover, inorganic oxidizingagents such as ammonium perchlorate, ammonium nitrate as well as metalpowder may also be admixed to these fuels in order to increase theenergy content thereof. Apart from the double-base" fuels, the so-calledcomposite fuels are also known, which are substantially based on the useof inorganic oxidizing agents with organic bonding agents. These bondingagents are for example polybutadiene, polyurethane, polysultide,polyester and the like. The oxidizing agents are mixed with the stillfluid organic components for producing these fuels and are subsequentlysubjected to a curing process, this resulting in a solid resilient massof predeterrninated fonn. Metal powder may also be added to these fuelsin order to increase the energy content thereof.

When using rockets for military purposes, it is in many cases necessarythat the propellants burn without the formation of smoke. This propertyexists, in the conventional fuels, on the basis of nitrocellulose andnitroglycerine, and, in the composite fuels, on ammonium nitrate. Thelimit of capacity, expressed as specific impulse, is about 230 to 240seconds in the double-base fuels, without the addition of ammoniumperchlorate and metal powder. However, in order to obtain suchcapacities, a fairly high proportion of nitroglycerine is necessary,which causes the fuel already to begin to deform plastically when theambient temperature is increased, this having a disadvantageous effecton combustion at high temperatures. However, an addition of ammoniumnitrate to the double-base fuels does not bring a substantial increasein energy, so that this possible solution can only be applied in rarecases. Composite fuels based on ammonium nitrate are only used forspecial purposes, e.g. for gas generators, since the adiabatic flametemperature is very low and therefore only specific impulses in theorder of 160 to I80 seconds can be achieved. All other fuels of thecomposite class produce a considerable amount of smoke duringcombustion, which smoke is composed, according to the choice of theoxidizing agent, of solid combustion products, e.g. potassium chloride,potassium oxide, sodium oxide and lithium chloride. When using ammoniumperchlorate as oxidizing agent, gaseous hydrochloric acid is producedwhich forms heavy fumes particularly when the atmosphere is damp. Thehydrochloric acid is also disadvantagous due to its corrosive effect onthe ground installa tion. The addition of metal powder, in particularaluminum or magnesium, also increases the formation of smoke since thecorresponding metal oxides are produced during combustion.

High-energy fuels which do not present these disadvantages of smokeformation and the corrosive effect on ground installations, may beproduced, on the basis of the present invention, from stabilizedpolyvinylnitrate. This purely synthetically produceable material hasgood thermoplastic properties and may be gelatinized with many organicnitric esters or other known plasticizers. In this form, it is capableof absorbing larger amounts of known explosives in pure or mixed form,such as for example trimethylenetrinitroamine, nitropenta, trotyl oreven ammonium nitrate. From such mixtures, propellants may be formed,which deliver completely smoke-less combustion products and highspecific impulses.

Polyvinyl nitrate was already under consideration in the 40's ascomponent in fuels and explosives, but its use in this field has, up tothe present, not been able to penetrate. The main disadvantage is statedto be the difficulty in the stabilization process. PVN decomposes moreor less quickly during storage and/or heating.

Polyvinylnitrate may be produced by nitration (esterification) ofpolyvinyl alcohol with pure concentrated nitric acid or with a so-calledmixed acid, consisting of concentrated nitric acid and concentratedsulfuric acid. Nitration (ester-iiication) is preferably carried out at0 C. The polyvinyl nitrate formed is thereby completely or partlysoluble in the nitrating acid depending upon the selected excessthereof. After nitration, the product is precipitated in water. Duringthis precipitation process, a considerable heat of dilution is produced,which causes the polyvinylnitrate precipitating in the water tocoagulate.

Although the particle size of the polyvinyl nitrate formed may berelatively well controlled according to the conditions of precipitation,an acid inclusion cannot be avoided. The conventional methods used inthe explosives and fuel industry for washing out the nitration productformed and subsequently treating with alkaline agents such as sodiumbicarbonate or ammonium bicarbonate and the like, do not lead to thetarget, since the included acid cannot be reached by the alkalis. It wasalso proposed to treat the polyvinylnitrate with an alkalinesolvent-water mixture and in this way to remove the residual acid. Also,this method does not produce good stability values. Finally, it is knownthat it was attempted to dissolve the polyvinylnitrate in acetone and toreprecipitate the lacquer solution after its neutralization in water.

In this way, good stability values may be obtained, but the process isonly to be carried out with considerable dilution and is thereforeuneconomical. The precipitate coagulates at higher concentrations.

It is therefore an object of this invention to procure a new andimproved process to prepare stabilized PVN from the crude product whichmay in turn be prepared by conventional means.

It is a further object of the invention to produce a stabilized PVNhaving outstanding properties of purity and ability to be stored forcomparatively long periods of time, substantially without decomposition.

Finally, it is another object of the invention to develop solid fuelcompositions superior to the known ones for use as propellants in rocketmotors.

It has now been found that pure and stabilized polyvinylnitrate, usuallywith a nitrogen content of more than 15 percent (theoretical maximumvalue about 15.7 percent), can be produced from the raw, decomposablePVN. The new stabilized polyvinylnitrate generally forms free flowing,non caking powders which was not possible until now.

Briefly spoken, the process according to the invention comprises thesteps of a. providing a solution of raw acid containing polyvinylnitratein an organic solvent,

b. neutralizing or rendering slightly alkaline said solution,

c. separating said solution from non-dissolved products of theneutralization,

d. introducing said purified solution of step (c) into a hotprecipitating bath in which said organic solvent is insoluble or onlypartially soluble, whereby said solvent is stripped and stabilizedpolyvinylnitrate is precipitated,and whereby the rate of introduction ofsaid purified solution corresponds to the stripping rate of said solventso that steady-state conditions are maintained, and whereby a solvent isselected in step (a) the pure boiling point of which or the azeotropicboiling point of which with the precipitating bath is below the boilingpoint of the pure precipitating bath.

As precipitating bath, either pure water or an aqueous medium whichcontains certain additives which are given hereinbelow by way ofexample, is preferably used. With aqueous precipitating baths, suchsolvents are preferably used in the PVN-solution which form with thewater an azeotrope with minimum boiling point, this usually beingimmediately the case when using solvents which are not or only partiallymiscible with water. Examples of such solvents are esters and ketones,such as ethyl and butyl acetate or methylethylketone andmethylisobutylketone. Ethyl acetate is preferred.

As PVN starting product, a direct product of nitration of polyvinylalcohol after precipitation with water, filtration and washing-out withwater until neutrality is reached, is advantageously used. This productcan immediately be dissolved in humid state without drying, e.g. inethylacetate.

One proceeds best by agitating the solution of the raw PVN with anaqueous solution of an alkaline agent, e.g. NaOH, NaHCO or Na CQ orother alkali metal hydroxides, carbonates or bicarbonates untilneutralization or weak basicity is reached, this necessitatingconsiderable amounts of the alkaline agent, according to the residualacid content which was included in the PVN particles.

The term weakly basizity" or slightly alkaline as used herein is to beunderstood as comprising pH values in the range of about 7 to about 9,preferably between about 7 and about 8.5.

When neutralization is finished, the neutralized or slightly alkalinePVN solution is separated from the salts formed, i.e. generally from thealkali-nitrates or their solution, if an aqueous neutralizing solutionwas used. To this end, the solution is generally allowed to stand andthe separation of the salts or the salt solution is awaited. Theseparation usually occurs by itself after a short time, the supernatantPVN solution has clarified and may be drawn off for further processing.If it ap pears advantageous, the PVN solution may be subjected to apost-sedimentation, e. g. by centrifugation.

From the beginning, such solutions are generally used which are good toprocess. For neutralization, a percent by weight solution of PVN inethyl acetate is for example particularly suitable. The content of thesolutions will vary depending upon the molecular weight or viscosity ofthe crude PVN. Too strongly diluted solutions represent volumes whichare difficult to handle, while too concentrated solutions are veryviscous and are difficult to neutralize.

For further processing of the solution, i.e. for the precipitation ofthe solutes, the solution may be diluted after neutralization.

Solutions which are more diluted than those described above generallyproduce purer products. After dilution, the solid content of thesolution is advantageously assayed, e.g. by evaporating a sample untilit is dry. Before precipitation, a stabilizer for the PVN may be addedto the solution, preferably 2- nitrodiphenylamine; however, the processaccording to the present invention mainly consists in a stabilization bypurifying the PVN.

The precipitating bath used consists, in the simplest case, of water.However, it has proved advantageous if the precipitating bath containscertain additives which increase the quality, i.e. in particular thestability, of the precipitating PVN. Also coming into question inparticular are surface active agents, preferably such of anion-activenature, e.g. the sodium salt of disulfonated dodecyldiphenyloxide;electrolytes, e.g. sodium sulfate; pH regulators, preferably foradjusting or maintaining slightly basic conditions, e.g. sodiumbicarbonate; protective colloids, e.g. bone glue, etc. If the PVNsolution contains a solvent or solvent mixture which is partiallymiscible with water, it is recommended to saturate the precipitatingbath with this solvent or mixture or to add so much to these liquidsthat saturation practically occurs at precipitation temperature. In thiscase, the boiling temperature of such a precipitating bath correspondsto the temperature at which the continuous precipitating processproceeds.

The precipitating bath is preferably strongly stirred at least duringthe introduction of the PVN solution, perhaps with the aid of avibrator-mixer. In this way, a uniform and fine precipitation isobtained.

It has proved advantageous to strip ofi about one-fifth of the PVNsolvent used per hour. After the end of the feed-in of the PVN solution,stripping is generally continued until the vapor temperature has risenby about in order to be sure that practically all solvent has beenstripped off.

Further processing may be undertaken in any way. One generally proceedsin the following manner: The precipitating bath is allowed to come toroom temperature, agitation being continued. Then the stirring device isswitched 05. The precipitated, stabilized PVN settles in the reactionvessel. The supernatant liquid is drawn off and replaced by the samequantity of water. Thereupon, stirring is again continued for about 1hour. Subsequently, the product is filtered off, washed and dried invacuo.

A polyvinylnitrate purified in this way, produced from polyvinyl alcoholwith a saponification number of 99 mole to which 0.5 percent2-nitrodiphenylamine were added, is thermally stable and satisfies forexample the Heat-Test according to US-MIL-Standard at 120 C. in 65 min.,at l34.5 C. in 25 min. and the Abel-Test at C. in 40 min.

The nitrogen content of such a product, determined according to theDumas method, gives 15.5 percent and corresponds to the theoreticalvalue in which the saponification number of the starting product and theproportion of stabilizer are taken into account.

This product has an explosion heat of 909 cal/g and a combustion heat of2,963 cal/g.

The process according to the present invention is further illustrated bythe following non-limiting Examples. All parts are by weight unlessotherwise stated.

EXAMPLE 1 20.4 parts of crude humid polyvinylnitrate, produced by thenitration (ester-ification) of a polyvinylalcohol with a saponificationnumber of about 99 mole and an approximate molecular weight of 100,000(Mowiol N -99 sold by Farbwerke Hoechst AG), subsequent dilution withwater and filtration (water content about 50 percent by weight) weredissolved in 40.8 parts of ethyl acetate. (The crude product containsnitrating acid still included therein. It would therefore be senselessto determine the nitrogen content.)

The solution obtained was mixed, with stirring, with a 5 percent byweight aqueous solution of NaHCO until the CO evolution had stopped andthe mixture had a pH-value of 7 to 7.5. To this end, 1.5 parts of theNaHCO solution were necessary. After the mixture had been allowed tostand for 20 minutes, two layers had formed. The upper organic layer wasseparated off. 1.0 parts thereof were dried in vacuo, and the residueamounted to 0.2 parts. The neutralized PVN solution was therefore still20 percent by weight. By the addition of 50 parts of ethyl acetate, anabout 10 percent by weight PVN solution was produced. 0.05 parts ofZ-nitrodiphenylamine were added to this solution.

A four-necked, round-bottomed flask with distilling attachment andcooler, thermometer, dropping funnel and agitator is charged with partsof an aqueous solution, which contains 1 part of disodiumdodecyldiphenyloxide disulfonate, 2.5 parts of sodium sulfate, 0.4 partsof sodium bicarbonate, 2 parts of bone glue and 5 parts of ethylacetate. The agitator (vibrator-mixer) is set into action and the flaskis heated until the contents thereof are boiling (about 71 C.).Immediately after the beginning of boiling, the above described PVNsolution is made to drop in. Heating of the flask and speed of feed-inare regulated so that the liquid level in the flask remains practicallyconstant. About 20 parts of an ethylacetate-water azeotrope with about92 percent ethylacetate and 8 percent water distil per hour with a vaportemperature of about 71 C. The precipitation of PVN is to be observed inthe flask. After 5 hours, the dropping in of the PVN solution is stoppedand distillation is continued until the vapor temperature has risen to100 C.; there are no more noteworthy amounts of ethyl acetate in thedistillate. The contents of the flask are cooled, while being stirred,to 25 C., the agitator device is switched off and the precipitated PVNis allowed to settle. The supernatant liquid is drawn off and replacedby 100 parts of de-ionized water. Stirring is continued for 1 hour atroom temperature, and the product is drawn off, washed with water anddried in vacuo. Yield: 9.4 parts of PVN (about 94 percent of the amountused).

EXAMPLE 2 26.0 parts of crude, humid polyvinylnitrate, produced by thenitration (esterification) of a polyvinylalcohol. with a saponificationnumber of about 98 mole and an approximate molecular weight of 41,000(Mowiol N 50-98" sold by Hoechst AG), subsequent dilution with water andfiltration (water content about 40 percent), were dissolved in 36.4parts of methylethylketone. The solution obtained was mixed, withstirring, in a 5 percent by weight aqueous NaHCO solution, until the CO,stopped forming and the mixture had a pH-value of 7. To this end, 2.5parts of the NaHCO solution were necessary. After the mixture was leftto stand for 2 minutes, two phases were formed. The upper organic phasewas separated off. 2.0 parts thereof were dried in vacuo. The residueamounted to 0.6 parts. The neutralized PVN-solution EXAMPLE 4Components, percent by weight W r 'lv, L, (l lVN AN ll'lN K. golr. (lnw.5

35 50 1.5 2,785.3 245. l 240. l 42 l l8 2,701). 7 2 15.3 2401 60 30 102,773. l 245. 8 240.3

EXAMPLE 10 Components, percent by weight n, (ll. PVN AN lhN (lvw.

EXAMPLE 6 Components, percent by weight 1100, Stability 6| PVN PETN RDXAN HTN Russ Pb.St. 7L min/s it 120 C ;I. 't111.

10 l 1 0. 4'2 12. 5 240 l. 645 10 l 1 0. 52 12. 4 230' 1.614 10 1 1 U.78 11.0 150 1. 551

parts of methylethylketone, a 15 percent by weight PVN solution wasproduced. 0.075 parts of 2-nitrodiphenylamine were added to thissolution.

A four-necked roundbottomed flask with distillating attachment andcooler, thermometer, dropping funnel and agitator is charged with 100parts of an aqueous solution which contains 1 part of disodiumdodecyldiphenyloxydisulfonate, 2.5 parts of sodium sulfate, 0.4 parts ofsodium bicarbonate, 2 parts of bone glue and 5 parts ofmethylethylketone. The agitator (vibrator-mixer) is set into action andthe flask is heated until the contents thereof are boiling (about 74 C).immediately after the beginning of boiling, the above described PVNsolution is made to drop in. Heating of the flask and rate of feed-inare controlled so that the liquid level in the flask remains practicallyconstant. About parts of a methylethylketone-water azeotrope with about88 percent MEK and 12 percent water are stripped per hour with a vaportemperature of about 74 C. The precipitation of PVN is to be observed inthe flask. After 5 hours, the dropping in of the PVN-solution is stoppedand distillation is continued until the vapor temperature has risen to100 C.: there are no more noteworthy amounts of MEK in the distillate.The contents of the flask are cooled, while being stirred, to C., theagitator device is switched off and the precipitated PVN is allowed tosettle. The supernatant liquid is drawn off and replaced by 100 parts ofde-salted water. Stirring is continued for 1 hour at room temperature,and the product is drawn ofi, washed with water and dried in vacuo.Yield: 14.4 parts of PVN (about 96 percent of the amount used).

Theoretically, water, as outer phase, may be replaced by another liquidcompound in which PVN is not soluble. Suitable solvents which form theinner phase with the dissolved PVN, should not or only partly be solublein the outer phase. Their boiling point must be lower than that of theouter phase.

The following examples illustrate the use of the stabilized PVN,obtained according to the present invention, in fuels. The parts are byweight.

In the above examples of application the abbreviations signify:

PVN: Polyvinlynitrate, stabilized according to the invention What weclaim is:

l. A process for preparing a stabilized polyvinylnitrate from a crudestarting material, comprising the steps of a. providing a solution ofraw acid containing polyvinylnitrate in an organic solvent,

b. neutralizing or rendering slightly alkaline said solution,

c. separating said solution from non-dissolved products of theneutralization, and

d. introducing said purified solution of step (c) into a hot aqueousprecipitating bath in which said organic solvent is insoluble or onlypartially soluble, whereby said solvent is stripped and stabilizedpolyvinylnitrate is precipitated, and whereby the rate of introductionof said purified solution corresponds to the stripping rate of saidsolvent so that steady-state conditions are maintained, and whereby asolvent is selected in step (a) the pure boiling point of which or theazeotropic boiling point of which with the precipitating bath is belowthe boiling point of the pure precipitating bath.

2. A process as in claim 1 in which step (b) is carried out by using anaqueous neutralizing solution of an alkali metal hydroxide, bicarbonateor carbonate.

3. A process as in claim 1 in which step (b) is carried out by using anaqueous neutralizing solution of NaHCO,,.

4. A process as in claim 1 in which in step (a) a solvent is usedselected from the group comprising an ester and a ketone at mostpartially miscible with water.

5. A process as in claim 1 in which in step (a) ethyl acetate is used asa solvent.

6. A process as in claim 1 further comprising the step of diluting thesolutions of step (b) with the same solvent as used in step a) 7. Aprocess as in claim 1 in which a solution of about 20 parts by weight ofcrude polyvinylnitrate in about 80 parts by weight of ethyl acetate isprepared in step (a), and further comprising the dilution of theneutralized solution of step (b) with ethyl acetate to a concentrationof about 10 percent by weight of polyvinyl nitrate.

8. A process as in claim 1, further comprising the step of adding tosaid solution after step (c) 2-nitrodiphenylamine as a stabilizingagent.

9. A process as in claim 1 in which the precipitating bath of step (d)is selected from the group comprising pure water and an aqueous mediumcontaining at least one additive of the group consistirifoTelectrolytes, protection colloids and pH regulators.

10. A process as in claim 1 in which an aqueous precipitating bath isused in step (d) which has been previously saturated with the organicsolvent of step (a) in the range between normal room temperature and theazeotropic boiling point.

11. A process as in claim 1, further comprising the step of vigorouslyagitating the precipitating bath of step ((1) during and after theprecipitating period.

12. A process as in claim 1, further comprising the step of continuingstripping off the solvent in step (d) after the feed of saidpolyvinylnitrate solution has been stopped until said precipitating bathis substantially solvent free.

4 IF t

2. A process as in claim 1 in which step (b) is carried out by using an aqueous neutralizing solution of an alkali metal hydroxide, bicarbonate or carbonate.
 3. A process as in claim 1 in which step (b) is carried out by using an aqueous neutralizing solution of NaHCO3.
 4. A process as in claim 1 in which in step (a) a solvent is used selected from the group comprising an ester and a ketone at most partially miscible with water.
 5. A process as in claim 1 in which in step (a) ethyl acetate is used as a solvent.
 6. A process as in claim 1 further comprising the step of diluting the solutions of step (b) with the same solvent as used in step (a).
 7. A process as in claim 1 in which a solution of about 20 parts by weight of crude polyvinylnitrate in about 80 parts by weight of ethyl acetate is prepared iN step (a), and further comprising the dilution of the neutralized solution of step (b) with ethyl acetate to a concentration of about 10 percent by weight of polyvinyl nitrate.
 8. A process as in claim 1, further comprising the step of adding to said solution after step (c) 2-nitrodiphenylamine as a stabilizing agent.
 9. A process as in claim 1 in which the precipitating bath of step (d) is selected from the group comprising pure water and an aqueous medium containing at least one additive of the group consisting of electrolytes, protection colloids and pH regulators.
 10. A process as in claim 1 in which an aqueous precipitating bath is used in step (d) which has been previously saturated with the organic solvent of step (a) in the range between normal room temperature and the azeotropic boiling point.
 11. A process as in claim 1, further comprising the step of vigorously agitating the precipitating bath of step (d) during and after the precipitating period.
 12. A process as in claim 1, further comprising the step of continuing stripping off the solvent in step (d) after the feed of said polyvinylnitrate solution has been stopped until said precipitating bath is substantially solvent free. 